The Silk Road made a small fortune during its two and a half years as the web's biggest anonymous black market for illegal drugs. As of Friday, at least one $28.5 million chunk of that fortune now belongs to the FBI.

An FBI official tells me that the bureau has located and seized a collection of 144,000 bitcoins, the largest seizure of that cryptocurrency ever, worth close to $28.5 million at current exchange rates. It believes that the stash belonged to Ross Ulbricht, the 29-year-old who allegedly created and managed the Silk Road, the popular anonymous drug-selling that site was taken offline by the Department of Justice after Ulbricht was arrested earlier this month and charged with engaging in a drug trafficking and money laundering conspiracy as well as computer hacking and attempted murder-for-hire.

The FBI official wouldn't say how the agency had determined that the Bitcoin "wallet"--a collection of Bitcoins at a single address in the Bitcoin network--belonged to Ulbricht, but that it was sure they were his. "This is his wallet," said the FBI official. "We seized this from DPR," the official added, referring to the pseudonym "the Dread Pirate Roberts," which prosecutors say Ulbricht allegedly used while running the Silk Road.

The FBI official pointed me towards this Bitcoin address, which according to the public Bitcoin transaction record known as the "blockchain" received transfers of close to 144,000 in just the last 24 hours. "They finished moving them at 3am this morning," said the official.

Sarah Meiklejohn, a researcher at the University of California at San Diego who has studied the Silk Road and Bitcoin, says she had been puzzling over the massive movement of bitcoins and confirms that several of the Bitcoin deposits that came together to create that wallet were tied to the Silk Road. "Addresses with 144,000 bit coins don't usually crop up overnight," she says.

When the Silk Road was taken down on October 1st, the FBI seized nearly 30,000 bitcoins, which are worth close to $6 million based on the recent surge in Bitcoin's value against the dollar. That was the first time the FBI had ever seized any amount of the cryptocurrency, according to the official I spoke to, so this more recent seizure represents by far the biggest Bitcoin takeover by law enforcement ever. "This is unprecedented," said the official. "Even if this were a regular drug case, it would be huge."

Even so, some of the Silk Road bitcoin profits may still be out of the FBI's hands. Last week, my Forbes colleague Kashmir Hill reported that another collection of around 111,000 bitcoins had been potentially tied to Ross Ulbricht--someone with the username "altoid," alleged to be Ulbricht in the criminal complaint against him, had mentioned the address in a 2011 post on the Bitcoin forum Bitcointalk.

That sum of 111,000 bitcoins at today's exchange rate would worth $22 million. But if that wallet were Ulbricht's, the FBI would need more than its Bitcoin address to seize the funds--They'd also still need to gain access to the private key that controls the wallet, which may be still stashed on a hidden computer somewhere. And as of Friday at noon, that sum hasn't moved from the address where "altoid" left it.

UC San Diego's Meiklejohn points out, however, that the FBI may have seized the key to that wallet and thus gained control of the bitcoins, which they could transfer at any time to the FBI's own wallet. "They may have the key and just be keeping them at that address," says Meiklejohn. "That would explain what we wouldn't have seen them move."

It's also possible that despite busting the Silk Road, the FBI has yet to find that key. Even after the biggest Bitcoin bust in history, some of the Dread Pirate Roberts' riches may still be out there.

Correction: An earlier version of this story stated that the FBI now controls 3.15% of all bitcoins. In fact the percentage is around 1.5%. Apologies for the error.

I'm a technology, privacy, and information security reporter and most recently the author of the book This Machine Kills Secrets, a chronicle of the history and future of information leaks, from the Pentagon Papers to WikiLeaks and beyond.